For example, US 2006/0222538 A1 (JP-A-2006-307829) discloses a high-pressure fuel pump having a plunger for pressurizing fuel in a compression chamber and supplying the fuel to an internal combustion engine. The fuel supplied to the internal combustion engine is injected from a fuel injection valve into a combustion chamber. The high-pressure fuel pump is provided with a discharge valve. The discharge valve is provided at the downstream of a compression chamber. The discharge valve opens to supply fuel from the compression chamber to the internal combustion engine when fuel pressure in the compression chamber becomes more than predetermined pressure. The discharge valve also operates as a check valve for restricting counterflow of fuel from the fuel injection valve of the internal combustion engine to the compression chamber.
When the fuel injection valve stops fuel injection in accordance with a fuel cut operation or stoppage of the internal combustion engines the downstream of the high-pressure fuel pump is blockaded between the discharge valve and the fuel injection valve. Alternatively, in a structure where a relief valve is provided for restricting fuel pressure from abnormally increasing in the downstream of the high-pressure fuel pump, the downstream of the high-pressure fuel pump is blockaded by the relief valve when the fuel injection valve stops fuel injection. In such structures, fuel pressure in the downstream of the high-pressure fuel pump is held high. The high pressure as control pressure is applied to fuel when the fuel injection valve stops. In addition, when the internal combustion engine is sufficiently warmed up, fuel pressure may be further increased by heat transmitted from the internal combustion engine to the fuel.
In such a case, fuel pressure in the downstream of the high-pressure fuel pump, i.e., in the upstream of the fuel injection valve may be held high when the fuel injection valve closes and the internal combustion engine stops. Accordingly, fuel may leak from a valve portion of the fuel injection valve into the combustion chamber of the internal combustion engine, which is being stopped. When fuel leaks into the combustion chamber during the internal combustion engine stops, a large amount of unburnt components such as HC may be discharged with exhaust gas in restarting of the engine. The amount of fuel injected from the fuel injection valve is preferably small when fuel injection is resumed under such a fuel cut operation. However, when fuel pressure in the upstream of the fuel injection valve is high, a large amount of fuel is injected from the fuel injection valve at the time of resuming the fuel injection. Consequently, a drive train of the internal combustion engine is applied with impact caused by rapid increasing in output power of the internal combustion engine.
In the high-pressure fuel pump of US 2006/0222538 A1, the outer wall of the housing of the discharge valve or the relief valve and the mount hole of the housing therebetween define a clearance. In the present structure, fuel is returned from the downstream of the high-pressure fuel pump to a low-pressure component through the clearance when the fuel injection valve stops, whereby fuel pressure in the downstream of the high-pressure fuel pump is reduced.
Here, the function to return fuel from the downstream of the high-pressure fuel pump to a low-pressure component during the stoppage of the internal combustion engine contradicts the function of the discharge valve. Accordingly, when the amount of fuel returning to the low-pressure component is large, the high-pressure fuel pump needs to discharge additional fuel correspondingly to the amount of the return fuel. As a result, loss in driving power of the pump increases. Thus, the return fuel is preferably small.
In addition, the housing of the discharge valve or the relief valve needs a minimum diameter, since the housing has a passage for flowing fuel and accommodates components such as a valve element. Accordingly, the present structure, in which fuel returns from the downstream of the high-pressure fuel pump to the low-pressure component by utilizing the clearance between the outer wall of the housing of the discharge valve or the relief valve and the mount hole, has a limitation in reduction in passage area of the clearance. That is, it is difficult to significantly reduce the amount of fuel returning to the low-pressure component.